Active RC filter circuit for use as a band-stop filter

ABSTRACT

A circuit forming an active RC filter for use as a band-stop filter in high and very-high frequency domains, having a second order transfer function, comprising a circuit that includes a building block for performing a filtering and amplifying function, and a building block S 3  for performing the summation V S  of the circuit output signal and the signal V E  applied to the input E of the circuit, characterized in that the filter building block is formed by a series arrangement of two filter building blocks F 1  and F 2  each having a first order all-pass function, and in that the circuit also includes an amplifier building block A 1  in a series arrangement with the filter building blocks F 1 , F 2 , inserted between the output of the latter building blocks and the output of the circuit, in that the input signal V E  is applied on the other hand to the summation building block S 3  through an amplifier A 2 , and in that the summation building block S 3  is formed by coupling the outputs of the amplifier building blocks A.sub. 1 and A 2  for forming the circuit output S at which the output signal V S  is available and in that each building block F 1  and F 2  comprises a phase-shifting bridge associated to an amplifier stage.

BACKGROUND OF THE INVENTION

The invention relates to a circuit forming an active RC filter to beused as a band-stop filter in the high and very-high frequency domains,having a second order transfer function, comprising a circuit thatincludes a building block for performing a filtering and an amplifyingfunction, and a building block for performing the summation V_(S) of thecircuit output signal and the signal V_(E) applied to the input of thecircuit.

The invention is applied to the realisation of integrated circuitsforming filters, in the high and very-high frequency domains, that canbe used, for example in one embodiment, as band-stop filters infrequency doublers for rejecting the unwanted signal at the fundamentalfrequency or, in another embodiment, as a band-stop filter havingdifferential inputs and outputs; or, in yet another embodiment, beparticularly suitable for a fine adjustment of the rejection.

An active all-pass filter is already known from the prior art from thepublication by J. TOW, in "IEEE Spectrum, December 1969, pp. 103-107",entitled "A Step-by-Step Active-filter Design".

This publication describes, among other things, an all-pass filterrealisation having a second order transfer function, which filter isshown in FIG. 8 of the said document. This circuit comprises threeoperational amplifiers arranged in series and is looped back. A fourthoperational amplifier performs the summation of the output signal of thefirst operational amplifier and the input signal of the filter circuit

The operational amplifiers that are used for constituting this circuithave a gain which is most certainly infinite at low frequencies, butwhich becomes very low at high and very-high frequencies. Therefore, thecircuit known from the said document has the disadvantage of not beingsuitable for use in these frequency domains.

Furthermore, the looped back circuit has the disadvantage to cause thecircuit to oscillate in certain conditions.

FIG. 6 of the said document also shows an active band-pass filter thathas similar characteristic features and thus exactly the samedisadvantages.

The circuits known from the said document further have the followingdisadvantages: on the one hand, they are formed by a large number oftransistors, which:

causes manufacturing to be costly in many cases

requires a large surface and is disadvantageous for use in integratedcircuits,

entails high consumption and is also disadvantageous for the use asmentioned above.

On the other hand, their characteristic frequency is not adjustable.

Finally, the capacitors used in this circuit have considerabledimensions and render this circuit not integrable

Therefore, the present invention has for its object to provide an activefilter circuit that allows to get rid of these disadvantages and thatspecifically:

can operate at high or very-high frequencies;

and in the case of the use at very-high frequencies is easy tointegrate, requires a small surface and has little consumption;

exhibits characteristic features so that the rejection frequency and therejection are adjustable and easy to control;

can admit a differential input signal and a differential output signal.

SUMMARY OF THE INVENTION

According to the invention, this object is achieved by means of acircuit as described in the preamble, characterized in that the filterbuilding block is formed by a series arrangement of two filter buildingblocks F₁ and F₂ each having a first order all-pass filter function, andin that the circuit also includes an amplifier building block A₁ in aseries arrangement with the filter building blocks F₁, F₂ insertedbetween the output of the latter building blocks and the output of thecircuit, in that the input signal V_(E) is applied on the other hand tothe summation building block S₃ through an amplifier A₂, and in that thesummation building block S₃ is formed by coupling the outputs of theamplifier building blocks A₁ and A₂ for forming the circuit output S atwhich the output signal V_(S) is available

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood by means of the followingdescription illustrated by the annexed drawings, in which:

FIG. 1 represents the building block diagram of the circuit according tothe invention;

FIG. 2a represents an embodiment of the circuit according to theinvention by means of field effect transistors;

FIGS. 2b and 2c, respectively, represent the curves of the relationshipbetween the output and input voltages of the circuit versus thefrequency, and of the phase shift φ+π of the output signal and the inputsignal versus the frequency;

FIG. 3 represents a variant of the circuit according to the invention;;

FIG. 3b represents another variant of the circuit according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In order to clarity the description of the invention it should first berecalled, that the transfer function of a second order all-pass activefilter is (p² +1)/(p² +aBp+1): see the Table on paqes 94-95 of thepublication entitled "Filtres Actifs" by Paul BILDSTEIN in the"Editionsde la Radio" (9 Rue Jacob, PARIS, FRANCE).

For an ideal second order band-stop filter this transfer function can bedescribed according to the relationship (1): of the Table I, in whichrelationship:

s is the Laplace variable that is derived from the variable p used inthe aforementioned publication (p being the lower Laplace variable) bythe relationship (2) of the Table I, also utilizing relationship (3) ofTable I.

The other parameters of the transfer function (1) are defined in theabovementioned publication where ω₁ is the rejection frequency for whichthe function (1) is absolutely F(jω₁)=0 and thus for which the rejectionis complete;

Q₀ is the quality factor or selectivity actor which indicates thesteepness of the slope of the curve |F(jω)|in the neighbourhood of ω₁ ;

ω₀ is the angular frequency associated to the poles of the transferfunction. Here it has been chosen to make the drive frequency ω₁correspond to the drive angular frequency ω_(p) of the above-referencedpublication, with in this publication ω₁ =ω₀.

G is the gain of the low-frequency circuit (if ω→0 and S→0, the factor Gis reduced to 1, and F(jω)→G).

The transfer function of a non-ideal circuit, but having a real fixedstructure, for which no requirements are made on the value of theelements, in the most simple case is of the second order and isexpressed by the relationship (4) of Table I.

For practical reasons of simplicity one is here interested in the casein which ω₁ =ω₀ whatever the values of the filter elements are, and inwhich,

Q₁ is the rejection factor,

|Q₁ |>Q₀ so that the filter operates in the band-stop mode, which leadsus to two cases:

    Q.sub.1 >Q.sub.0

and

    Q.sub.1 <=Q.sub.0.

Thus, there appears a novel filter parameter which is the rejection R ofthe filter provided by (he relationship (5) of the Table I, if assumingthat ω₁ ≃ω₀.

The greater the absolute value of Q₁ (denoted Q₁), the better is therejection R. At the limit value the rejection R is complete when Q₁ isinfinite.

Well, the real band-stop filters can be subdivided into two categories:

the first category corresponds to the case in which Q₁ is finite, andgreater than 0 (Q₁ >0) whatever the filter elements. In theseconditions, the filters cannot realise a total rejection. The maximumvalue Q₁ is the one that determines the filter limits. Actually, for agiven value Q₁, if the value Q₀ enhances, the rejection R is diminished.

the second category corresponds to the case in which the fact that Q₁ isgreater or smaller than 0 contrarily depends on the filter elements. Inthis case there are particular element values such as (1/Q₁)=0, wherethe rejection R is complete. The transfer function G(s) provided by therelationship (4) is thus reduced to the function F(s) provided by therelationship (1).

The filters belonging to the second category especially have theproperty of being realised by choosing the elements such that Q₁ =-Q₀(and always ω₁ =ω₀), so that the function (4):

    |G(jω)|=G is constant.

Thus, the filter becomes of the second order all-pass type. The phasedifference between the output signals and input signals passes from 0 to-2π, when ω varies from 0 to infinity.

The main object of the invention is to realize active filters comprisingthe least possible number of elements in order to promote integration,and operating at high and very-high frequencies.

To this end, the integrated circuits of the filters according to theinvention are advantageously realized with gallium arsenide (GaAs).

Furthermore, according to the invention, one has chosen to realiseactive band-stop filters of the second category, which permits to attainthe following advantages:

a considerable and easily controllable rejection R

an equally large selectivity as that of passive filters known from thestate of the art mentioned hereinafter;

a D C. adjustable rejection factor Q₁.

The filters according to the invention will be advantageously used infrequency doublers for rejecting the unwanted signal at the fundamentalfrequency. The circuit according to the invention though being of thesecond category will not be used as a second order all-pass filter.Actually, it will be noticed that the filter itself is realised by meansof all-pass filter elements each of the first order in an embodimentdescribed hereinafter.

On the other hand, also known from the state of the art, there areband-stop filters of the second category operating at high frequenciesBut these circuits are realised by means of passive elements. Forexample, the circuit commonly referred to as "Robinson bridge" alsoknown by the name of "Wien bridge", and represented on page 121 of thereference publication entitled "Electric Filters"by T. H. TURNEY (Ed.Pitman and Sons, London, England 1945) could be considered. Thiscircuit, although it is capable of operating at very-high frequencies,has various disadvantages, specifically:

an output that has to be differential,

a high output impedance,

a zero gain, because it is realised by means of passive elements,

generally speaking, difficult control and poor performance

The block diagram of the band-stop filter circuit according to theinvention is illustrated in FIG. 1. This circuit is formed by two partsdefined by dashed lines. The first part is formed by the building blocksF₁ and F₂ each having the function of an all-pass filter, and

the second part AS this the function of a summing amplifier.

The blocks F₁ and F₂ have for their transfer function the resPectiverelationships (6) and (7) of Table I.

The amplifier block AS is formed by an amplifier A₁ having gain k₁ andan amplifier A₂ having gain k₂. The amplifier A₁ is arranged in cascadewith the filter blocks F₁ and F₂ while amplifier A₂ is arranged inparallel with the branch formed by F₁, F₂ and A₂. The adder S₃ sums theoutput signals of the amplifiers A₁ and A₂, and furnishes the outputsignal V_(S) of the circuit according to the invention, whose inputsignal V_(E) is applied to the junction of the filter block F₁ input andthe amplifier A₂.

Consequently, the transfer function of the whole circuit of FIG. 1 isG(s), provided by the relationship (4), the parameters being defined bythe relationships (8), (9), (10), (11) and (16).

In these relationships:

τ₁ and τ₂ are the respective time constants associated to the poles ofthe filter blocks F₁ and F₂ ;

τ'₁ and τ'₂ are the respective time constants associated to the zeroesof the filter blocks F₁ and F₂.

It should be observed that in the normal case:

    τ'.sub.1 =τ.sub.1

    τ'.sub.2 =τ.sub.2

    and ω.sub.1 =ω.sub.0.

It should also be observed that the maximum Q₀ =1/2 is obtained for

    τ.sub.1 =τ.sub.2.

The circuit according to the invention thus forms a bandstop filter ofthe second category defined above, which can realise a total rejection.

Embodiment I

A first embodiment of the circuit according to the invention is shown inFIG. 2a. This circuit of FIG. 2a is a basic version of the circuitaccording to the invention, which permits several variants whosecharacteristic features are shown in the other embodiments.

In the version of FIG. 2a, the all-pass (AP) filter blocks areidentical. They each comprise a phase-shifting bridge, which is followedby a differential PUSH-PULL amplifier stage.

Each phase-shifting bridge comprises between the opposite nodes 11, 12and 21, 22, for filters F₁ and F₂, respectively, in a first branch aresistor R₁₁, R₂₁, and a capacitor C₁₁, C₂₁ ; and in a second branch acapacitor C₁₂, C₂₂ and a resistor R₁₂, R₂₂. The output signal of eachphase-shifting bridge is available at the junctions 13, 14 and 23, 24for filters F₁ and F₂ respectively, of the resistors and capacitors ofeach branch.

The resistors R₁₁, R₁₂, R₂₁, R₂₂ of the phase-shifting bridges areactually, as is represented in the FIGS. 2a, 3a, 3b, the respectivefield effect transistors (MESFET) T₁₁, T₁₂ for the phase-shifting bridgeof the filter block F₁, and T₂₁, T₂₂ for the phase-shifting bridge ofthe filter block F₂. The source and the drain of these transistors formthe ends of the resistors, and their gate receives a bias voltage E₆.These resistors are thus adjustable by controlling this voltage E₆ (seeFIG. 2a). This control allows setting of the rejection frequency f₁ ofthe circuit according to the invention. A resistor arranged in serieswith E₆ protects the transistors T₁₁, T₁₂, T₂₁, T₂₂ and slightly enhancef₁.

The maximum rejection frequency f₁ max for the rejection R correspondsto a zero gate-source voltage (V_(GS) =0) for the transistors of thephase-shifting bridges, which is obtained for:

    E.sub.6 maximum=E.sub.2 =2.5V

for the example described above (see FIG. 2a).

The minimum frequency f₁ min for the rejection R corresponds to theminimum voltage chosen as a function of the threshold voltage of thetransistor that can be applied between the gate and source of thetransistors of the phase-shifting bridge, that is to say:

V_(GS) min=-1.8V, which is obtained for E₆ min=E₂ -1.8V-0.7V in thisembodiment.

The ratio of the output voltages V_(SP) of the nodes 13, 14 or 23, 24 tothe input voltages V_(EP) of the nodes 11, 12 or 21, 22 of thephase-shifting bridges is provided by the relationship (12) of Table I,in which relationship R₀ and C₀ are the resistances and capacitances ofthe different elements of the phase-shifting bridges.

Each filter block F₁ or F₂, having the function of an all-pass filter,subsequently comprises a differential amplifier stage of which eachbranch is of the PUSH-PULL type.

These amplifiers, as they are represented in FIG. 2a, comprise in aseries arrangement a first branch comprised of a transistor at the top(T₁₃ and T₂₃ for filters F₁ and F₂, respectively,) L- and a transistorat the bottom (T₁₄ and T₂₄ for filters F₁ and F₂, respectively). Thisfirst branch is arranged in parallel with a second branch comprised of atransistor at the top (T₁₅ and T₂₅, respectively) and in series with atransistor at the bottom (T₁₆ and T₂₆, respectively).

The transistors at the top have common drains which are connected to aD.C. supply voltage E₁. The transistors at the bottom have commonsources which are connected to ground.

The gates of the transistors at the top are connected to the respectiveoutput junctions of the phase-shifting bridge that precedes theamplifier stage, these junctions also being cross-connected to the gatesof the transistors at the bottom of the same amplifier stage.Furthermore, the gates of the transistors at the top are biased througha resistor with a D.C. supply voltage E₂ (R₁₃ for T₁₃), (R₁₅ for T₁₅),(R₂₃ for T₂₃) and (R₂₅ for T₂₅). The gates of the transistors at thebottom are also biased with a voltage E₅ and E₄, respectively, for thefilters F₁ and F₂. Isolating capacitors (C₁₃, C₁₄ for F₁ and C₂₃, C₂₄for F₂, respectively) are inserted into the cross connections of thetransistor gates at the top and at the bottom.

The output signal of each differential amplifier stage occurs betweenthe junctions of the transistors at the top and at the bottom of eachPUSH-PULL branch. The output signal of the amplifier stage filter F₂occurs at the junction 25 between the transistors T₂₃ and T₂₄.

On the other hand, the input signal V_(E) of the circuit is applied tothe junction 11 of the first phase-shifting bridge through an isolatingcapacitor C₁₁₀, the junction 12 receiving a bias voltage E₂.

The signals the outputs 13 and 14 of the first phase-shifting bridge areapplied to the gates of the transistors 13 and 15 of at the top of theamplifier stage of F₁ and to the gates of the transistors T₁₆ and T₁₄ atthe bottom of this same amplifier. The signals of the outputs 15 and 16of this amplifier are applied through isolating capacitors C₂₁₀ and C₂₂₀to the inputs 21 and 22 of the phase-shifting bridge of block F₂. Thesignals of the outputs 23 and 24 of this phase-shifting bridge areapplied to the gates of the transistors T₂₃ and T₂₅ at the top and tothose of the transistors T₂₄ and T₂₆ at the bottom, cross-connected tothe amplifier stage of F₂. The signal of the output 25 of this amplifierstage is available at the node of the transistors T₂₃ and T₂₅.

The transistors forming the PUSH-PULL differential amplifier stages areall field effect transistors.

The bias voltage E₄ applied to the gates of the transistors at thebottom of the amplifier stage F₂ is used for a coarse adjustment of therejection R of the whole circuit, while the bias voltage E₅ applied tothe gates of the transistors at the bottom of amplifier stage of F₁ isused for a fine adjustment of this rejection R.

The circuit according to the invention furthermore comprises anamPlifier A₁ arranged in series with the filter blocks F₁ and F₂. Thisamplifier A₁, as is represented in FIG. 2a, is formed by the fieldeffect transistor T₃₁.

The source of this transistor T₃₁ is connected to ground and its drainis connected to the supply voltage E₁ via a charge. This charge iscomprised of the transistor T₃₀ whose drain is connected to the supplyvoltage E₁, whose gate is biased over a resistor R₃₀ by the voltage E₂,its gate also being connected to its source across the capacitor C₃₀.The output signal of this amplifier stage A₁ is available at the drainof the transistor T₃₁.

The gate of the amplifier transistor T₃₁ (input of the amplifier A₁)receives across a capacitor C₃₁₀ the output signal of the PUSH-PULLamplifier stage of F₂ taken off at the node 25 of the transistors T₂₃,T₂₄. On the other hand, this gate is biased by the voltage E₁ through aresistor R₃₁.

The bias voltage E₄ simultaneously adjusts the gain G₂ of the filterblock F₂ and the gain k₁ of the amplifier A₁ ; thus, it coarsely adjuststhe rejection factor Q₁.

Conversely, the bias voltage E₅, which is supplied to the gates of thetransistors at the bottom of the PUSH-PULL amplifier stage of filterblock F₁, performs a fine adjustment of the gain of this amplifier stageand thus a fine adjustment of the rejection factor Q₁.

In this embodiment we choose:

E₁ =6V

E₂ =2.5V

E₄ will be adjusted for obtaining:

E₅ ≃-0.8V.

It should be observed that E₂ can be replaced by a resistance bridgeinserted between E₁ and earth for diminishing the required number ofvoltage sources.

The circuit according to the invention also comprises an amplifier A₂having gain k₂. This amplifier is formed by a field effect transistorT₃₂ arranged in parallel and thus admitting the same charge as thetransistor T₃₁ (A₁). The amplifier transistor T₃₂ receives on its gate(input A₂) the input siqnal V_(E) across an isolating capacitor C₃₂₀.

The common drains T₃₁ and T₃₂ perform the function of the summationelement S₃ (see FIG. 1). Besides, the gate of the amplifier transistorT₃₂ (A₂) the input signal V_(E) across an isolating capacitor C₃₂₀.

The common drains T₃₁ and T₃₂ perform the function of the summationelement S₃ (see FIG. 1). Besides, the gate of the amplifier transistorT₃₂ (A₂) is biased by a voltage E₃. This bias voltage is to remainfixed, but its value need not necessarily be very accurate (E₃ ≃-0.8V)should be used.

The arrangement represented in FIG. 2a adds a phase shift π to theoutput signal V_(S) available at the common drain of the transistors T₃₁(A₁) and T₃₂ (A₂) when compared to the block diagram represented in FIG.1.

For a bias voltage E₆ of the transistor gates of the phase-shiftingbridges so that:

E₆ =2.5, the rejection frequency is f₁ =3.23 GHz,

E₆ =0.7V, the rejection frequency is f₁ =0.90 GHz.

the frequency band covered is very large.

On the other hand, if the bias voltage E₅ is adjusted with an accuracyof ±25 mV, the circuit according to the invention will guarantee:

a rejection R≧56 dB at f₁ =3.23 GHz,

a rejection R≧60 dB at f₁ =0.90 GHz.

These results are easily obtained, because the circuit according to theinvention is particularly easy to adjust. Actually, it is simple toadjust the gain of each PUSH-PULL amplifier stage used. Therefore, ineach building block F₁ and F₂, especially these amplifiers of thePUSH-PULL type have been chosen in preference to the classical invertedamplifiers.

It should also be observed that as the gain of the PUSH-PULL is adjustedbetter at the lowest frequencies, the rejection there is the best.However, it still remains very high. The low frequency gain of the wholecircuit at the lowest rejection frequencies f₁ is of the order of 12 dB.It is 10 dB at the highest frequencies f₁.

The rejection frequency;

f₁ =3.23 GHz is obtained for E₄ =-1V and E₅ ≃-0.85V

f₁ =0.90 GHz is obtained for E₄ =-1.6V and E₅ ≃-0.95V.

The FIGS. 2b and 2c present by way of example the computerized simulatedcharacteristic values of the ratios of the signals |V_(S) /V_(E) | in dBas a function of the operating frequency, which is expressed in GHz; andthe value of the phase shift φ of the signals V_(S) and V_(E) as afunction of the frequency expressed in GHz, for E₄ =-1V, E₅ =-0.85V, E₆-2.5V.

The circuit according to the invention specifically offers the advantageto make f₁ adjustable, resulting in:

(a) the possibility to correct the value of f₁, of which the integratedcircuits always produce dispersed values, owing to the dispersion of thecharacteristic values of the elements in the various circuits;

(b) the possibility to use the same circuit with many differentfrequencies.

The Table II presents by way of a non-exhaustive example, values of theelements for the circuit as shown in FIG. 2a, with field effecttransistors normally of the band-pass type in the absence of gate-sourcebias, presenting a threshold voltage of the order of:

V_(T) =-2.5V and a gate length 1=0.7 μm.

The circuit according to the invention is realized by means of elementsof Table II on a gallium arsenide substrate (GaAs).

By taking again equation (4) which, as will have been noticed, aPPliesto the present circuit, it has already been observed hereinbefore that:First, the filter circuit according to the invention is of the band-stopfilter type if the relationship (13) is satisfied.

The structure according to the invention enables to attain that Q₁ ispositive or negative and the relationship (14) satisfied. Secondly thecircuit according to the invention also enables to attain that Q₁ =-Q₀,which corresponds to the case in which the circuit has the function ofan all-pass filter. In addition, the circuit according to the inventionallows to pass from the function of a band-stop filter to the functionof an all-pass filter with the same elements.

Actually, it will be sufficient to make the gate bias voltages of thetransistors of the different stages vary, to attain the variation

either of k₁,

or of k₂,

or of G₁

or of G₂

Then in a continuous manner we can pass from:

Q₁ =-∞(band-stop) to Q₁ =-Q₀ (all-pass).

Embodiment II

The second embodiment of the circuit according to the invention is shownin FIG. 3a. The circuit of FIG. 3a is a version that allows a fineradjustment of the gain and thus of the rejection. Consequently, theamplifiers of blocks F₁ and F₂ are not identical.

The amplifier of filter block F₁ does not have the gates of the top andbottom transistors cross-connected, and thus is not of the PUSH-PULLtype. It is simply comprised of two followers of which the gain issmaller than in embodiment I, but which can be adjusted more accurately.Thus, the rejection can be adjusted in a simpler manner.

Thus, a choice may be made between the circuit of embodiment I and thatof embodiment II according to the intended use.

In this embodiment II a D.C. voltaqe E₅ ≃0 is now used, whilemaintaining therefor negative values (E₅ <0). The values of thecapacitors of the phase-shifting bridges are different from those of theembodiment 1 as is shown in Table lII.

The rejection frequencies obtained are:

F₁ =3.42 GHz, for E₆ =2.5V

f₁ =0.945 GHz, for E₆ =0.7V.

The band covered by the frequency f₁ in the circuit of the embodiment IIis also a little larger than that of embodiment I.

If the voltage E₅ is adjusted to ±25 mV, it is guaranteed that:

R≧59 dB with f₂ =3.42 GHz

R≧61 dB with f₁ =0.945 GHz.

The gain at the lowest frequencies is equal to 7.5 dB because thefollowers show less gain than the PUSH-PULL of the previous embodiment,as has already been stated.

To obtain the rejection frequencies

f₁ =3.42 GHz, E₄ is to be equal to -0.85V, E₅ ≃-0.19V

f₁ =0.945 GHz, E₄ is to be equal to -1.85V, E₅ ≃-0.28V.

Embodiment III

This embodiment allows differential operation (see FIG. 3b).

The differential inPut signal V_(E) =V_(E2) -V_(E1) is applied betweenthe nodes 11 and 12 of the phase-shifting bridge of the first buildingblock F₁.

The output signal can be differential: this will be V_(S) =V_(S1)-V_(S2), the voltage V_(S1) being available at node 25 of thedifferential amplifier of the second filter block F₂ and the voltageV_(S2) at the node 26. The output signal can also be obtained at groundreference; then one of the voltages should be used which is availableeither at node 25 or at node 26.

The amplifier A₁ is combined with the PUSH-PULL amplifier stage of blockF₂. Thereto, the gates of the bottom transistors T₂₄, T₂₆ of this stageare biased by the same D.C. voltage E₄ as the one that biases the gatesof the bottom transistors T₁₄, T₁₆ of the PUSH-PULL amplifier stage offilter block F₁. This allows coarse adjustment of the rejection of thewhole circuit by adjusting E₄.

The amplifier A₂ is realized by adding field effect transistors T'₂₃,T'₂₄, T'₂₅, T'₂₆ in a parallel arrangement to the respective transistorsT₂₃, T₂₄, T₂₅, T₂₆ of the PUSH-PULL amplifier of filter block F₂. Thegates of the top transistors T'₂₃, T'₂₅ are cross-connected by means ofcapacitors C'₂₆, C'₂₃ and C'₂₅, C'₂₄ to the gates of the respectivebottom transistors T'₂₆, T'₂₄.

On the other hand, these cross-connected gates receive the differentialinput signal. Then, the gates T'₂₃ and T'₂₆ are connected to theterminal 11 through the D.C. isolating capacitor C₁₁₀, and the gates ofthe transistors T'₂₅ and T'₂₄ are connected to the terminal 12 via theD.C. isolating capacitor C₁₂₀.

The gates of the transistors at the top T'₂₃ and T'₂₅ are also biased bythe D.C. voltage E₂ through the resistors R'₂₃ and R'₂₅, while thebottom transistors T'₂₄, T'₂₆ are biased by the D C voltage E₅ throughthe resistors R'₂₄, R'₂₆ for the fine adjustment of the rejection.

Table III presents by way of a non-exhaustive example the values of theelements for the use of the circuit of FIG. 3b.

Adjusting the frequency f₁ is always effected by having the gates of thetransistors of the phase-shifting bridges biased by E₆.

The summation S₃ is realized because the transistors of amplifiers A₁and A₂ are arranged in parallel.

The rejection frequency:

f₁ =3.85 GHz is obtained for E_(6max) =E₂ =2.5V

f₁ =0.906 GHz is obtained for E_(6max) =E₂ -1.8V=0.7V.

The frequency band covered by f₁ is larger than in the embodimentsdescribed hereinbefore. This is because when the frequency f₁ is higher,the amplifier stages produce a phase delay leading to a diminishing ofthe frequency f₁. Since in this embodiment III the structure is suchthat the complete circuit comprises one stage less than in theembodiments I and II, the frequency f₁ is less affected by the phaseshift caused by the amplifier stages.

If E₅, which is the fine adjustment of the rejection, is controlled to±25 mV, it is guaranteed that:

R≧53 dB with f₁ =3.85 GHz

R≧51 dB with f₁ =0.906 GHz.

The gain at the lowest frequencies is of the order of 1 dB if thedifferential output signal V_(S) =V_(S1) -V_(S2) is used at the nodes 25and 26, and approximately -5 dB if an asymmetrical output signal V_(S1)or V_(S2) is used at 25 or 26.

On the other hand, the rejection frequency f₁ =3.85 GHz is obtained forE₄ =-0.8V, E₅ ≃-1.09V f₁ =0.906 GHz is obtained for E₄₌₋₁.95 V, E₅≃-1.1V with the values of the elements of Table IV.

                  TABLE I                                                         ______________________________________                                         ##STR1##                     (1)                                             p = s/ω.sub.1           (2)                                             s = jω                  (3)                                              ##STR2##                     (4)                                              ##STR3##                     (5)                                              ##STR4##                     (6)                                              ##STR5##                     (7)                                             ω.sub.0 = (τ.sub.1 τ.sub.2).sup.-1/2                                                          (8)                                              ##STR6##                     (9)                                              ##STR7##                     (10)                                            G = k.sub.1 + k.sub.2 G.sub.1 G.sub.2                                                                       (11)                                             ##STR8##                     (12)                                            |Q.sub.1 | > Q.sub.0                                                                      (13)                                            1/Q.sub.1 = 0                 (14)                                             ##STR9##                     (16)                                            ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        (FIG. 2a)                                                                     Resistors                                                                             Capacitors  Transistors Supply                                        R     kQ    C.sub.11 pF   t (λ)                                                                         μm                                                                              E     V                                 ______________________________________                                        R.sub.13                                                                            10    C.sub.11 0.13 T.sub.11                                                                             10   E.sub.1                                                                              6                                R.sub.15                                                                            10    C.sub.12 0.13 T.sub.12                                                                             10   E.sub.2                                                                              2.5V                             R.sub.14                                                                            10    C.sub.13 1    T.sub.13                                                                             40   E.sub.3                                                                             -0.8                              R.sub.16                                                                            10    C.sub.14 1    T.sub.14                                                                             40   E.sub.4                                 R.sub.23                                                                            10    C.sub.21 0.13 T.sub.15                                                                             40   E.sub.5                                 R.sub.25                                                                            10    C.sub.22 0.13 T.sub.16                                                                             40   E.sub.6                                 R.sub.24                                                                            10    C.sub.23 1    T.sub.21                                                                             10                                           R.sub.26                                                                            10    C.sub.24 1    T.sub.22                                                                             10                                           R.sub.30                                                                            10    C.sub.30 1    T.sub.23                                                                             40                                           R.sub.31                                                                            10    C.sub.310                                                                              1    T.sub.24                                                                             40                                           R.sub.32                                                                            10    C.sub.320                                                                              1    T.sub.25                                                                             40                                                       C.sub.110                                                                              1    T.sub.26                                                                             40                                                       C.sub.210                                                                              1    T.sub.30                                                                             25                                                       C.sub.220                                                                              1    T.sub.31                                                                             35                                                                     T.sub.32                                                                             10                                           ______________________________________                                         E.sub.4 = coarse adjustment of rejection R                                    E.sub.5 = fine adjustment of rejection R                                      E.sub.6 = adjustment of rejection frequency f.sub.1                      

                  TABLE III                                                       ______________________________________                                        (FIG. 3a)                                                                     Resistors                                                                             Capacitors  Transistors Supply                                        R     kQ    C.sub.11 pF   T (λ)                                                                         μm                                                                              E     V                                 ______________________________________                                        R.sub.13                                                                            10    C.sub.11 0.12 T.sub.11                                                                             10   E.sub.1                                                                              6                                R.sub.14                                                                            10    C.sub.12 0.12 T.sub.12                                                                             10   E.sub.2                                                                              2.5V                             R.sub.15                                                                            10    C.sub.21 0.12 T.sub.13                                                                             40   E.sub.3                                                                             -0.8                              R.sub.16                                                                            10    C.sub.22 0.12 T.sub.14                                                                             25   E.sub.4                                 R.sub.23                                                                            10    C.sub.23 1.-  T.sub.15                                                                             40   E.sub.5                                 R.sub.24                                                                            10    C.sub.24 1.-  T.sub.16                                                                             25   E.sub.6                                 R.sub.25                                                                            10    C.sub.30 1.-  T.sub.21                                                                             10                                           R.sub.26                                                                            10    C.sub.31 1.-  T.sub.22                                                                             10                                           R.sub.30                                                                            10    C.sub.32 1.-  T.sub.23                                                                             40                                           R.sub.31                                                                            10    C.sub.110                                                                              1.-  T.sub.24                                                                             40                                           R.sub.32                                                                            10    C.sub.210                                                                              1.-  T.sub.25                                                                             40                                                       C.sub.220                                                                              1.-  T.sub.26                                                                             40                                                                     T.sub.30                                                                             45                                                                     T.sub.31                                                                             10                                                                     T.sub.32                                                                             70                                           ______________________________________                                         E.sub.4  = coarse adjustment of rejection R                                   E.sub.5 = fine adjustment of rejection R                                      E.sub.6 = adjustment of rejection frequency f.sub.1                      

                  TABLE IV                                                        ______________________________________                                        (FIG. 3b)                                                                     Resistors  Capacitors  Transistors  Supply                                    R      kQ      C.sub.11                                                                             pF     T (λ)                                                                        μm  E   V                               ______________________________________                                        R.sub.13                                                                             10      C.sub.11                                                                             0.13   T.sub.11                                                                            10     E.sub.1                                                                           6                               R.sub.14                                                                             10      C.sub.12                                                                             0.13   T.sub.12                                                                            10     E.sub.2                                                                           2.5                             R.sub.15                                                                             10      C.sub.13                                                                             1      T.sub.13                                                                            40     E.sub.4                             R.sub.16                                                                             10      C.sub.14                                                                             1      T.sub.14                                                                            40     E.sub.5                             R.sub.23                                                                             10      C.sub.21                                                                             0.13   T.sub.15                                                                            40     E.sub.6                             R'.sub.23                                                                            10      C.sub.22                                                                             0.13   T.sub.16                                                                            40                                         R.sub.24                                                                             10      C.sub.23                                                                             1      T.sub.21                                                                            10                                         R'.sub.24                                                                            10      C.sub.24                                                                             1      T.sub.22                                                                            10                                         R.sub.25                                                                             10      C'.sub.23                                                                            1      T.sub.23                                                                            30                                         R'.sub.25                                                                            10      C'.sub.24                                                                            1      T'.sub.23                                                                           10                                         R.sub.26                                                                             10      C'.sub.25                                                                            1      T.sub.24                                                                            30                                         R'.sub.26                                                                            10      C'.sub.26                                                                            1      T'.sub.24                                                                           10                                                        C.sub.110                                                                            1      T.sub.25                                                                            30                                                        C.sub.120                                                                            1      T'.sub.25                                                                           10                                                        C.sub.210                                                                            1      T.sub.26                                                                            30                                                        C.sub.220                                                                            1      T'.sub.26                                                                           10                                         ______________________________________                                         E.sub.4 = coarse adjustment of rejection R                                    E.sub.5 = fine adjustment of rejection R                                      E.sub.6 = adjustment of rejection frequency f.sub.1                      

What is claimed is:
 1. An active RC filter circuit, comprising:a firstfilter receptive of the filter circuit input signal and having a firstorder all-pass filter function; a second filter having a first orderall-pass filter function connected in series with said first filter; afirst amplifier connected for receiving the output of said secondfilter; a second amplifier connected for receiving the input of theactive filter circuit; and summing means for summing the respectiveoutputs of said first and second amplifiers for developing the output ofthe active filter circuit.
 2. An active RC filter circuit according toclaim 1, wherein said summing means is comprised of means for couplingthe respective outputs of said first and second amplifiers.
 3. An activeRC filter circuit according to claim 2, wherein each of said first andsecond filters comprise a phase-shifting bridge circuit and an amplifierfor amplifying the output of said bridge circuit.
 4. An active RC filtercircuit according to claim 1, wherein each of said first and secondfilters comprise a phase-shifting bridge circuit and an amplifier foramplifying the output of said bridge circuit.
 5. An active RC filtercircuit according to claim 1, wherein each of said first and secondfilters comprisea phase-shifting bridge circuit, said phase-shiftingbridge circuit comprising a pair of parallel branches between two nodesdefining the input nodes of said bridge circuit, a first of saidbranches comprising a first resistor and a first capacitor in seriesbetween said bridge input nodes, the second of said branches comprisinga second capacitor and a second resistor in series between said bridgeinput nodes and in an order opposite that of said first capacitor andsaid first resistor, the respective capacitor and resistor comprisingsaid first branch and said second branch joining at a node, and thenodes of each branch together defining the output of said phase-shiftingbridge circuit; and an amplifier for amplifying the output signal fromsaid phase-shifting bridge.
 6. An active RC filter circuit according toclaim 5, wherein said first resistor and said second resistor areadjustable in value for tuning the rejection frequency of the filtercircuit.
 7. An active RC filter circuit according to claim 5, whereinsaid first resistor and said second resistor are comprised of fieldeffect transistors having gate regions which are biasable for changingthe resistance thereof for tuning the rejection frequency of the filtercircuit.
 8. An active RC filter circuit according to claim 5, whereinsaid amplifier is a differential amplifier.